1. Field of the Invention
The present invention relates to a microcellular mobile radio communication system.
2. Description of the Prior Art
A mobile communication system such as, for example, an automobile telephone system has radio base stations which are composed of an antenna, an antenna duplexer, transmitter and receiver devices for communication and control, a base station controller and other device, as shown in FIG. 11.14 on page 252, of AUTOMOBILE TELEPHONE edited by Moriji Kuwabara and published by the Institute of Electronics, Information and Communication Engineers.
An automobile telephone system has its service area divided into a plurality of small zones or regions called microcells each having its own radio base station. A plurality of radio base stations in noncontiguous microcells can reuse the same radio frequencies without undue interference therebetween, for effective utilization of limited frequency resources. Such an automobile telephone system is referred to as a microcellular mobile radio communication system. In microcellular mobile radio communication systems, it may become necessary for the radio base stations to be positioned in scattered locations, sometimes on street lampposts and utility poles, for example.
Current radio base stations which include transmitter and receiver devices and a base station controller, among others, as described above, cannot satisfy the requirements for small size and high reliability which must be met when installed on street lampposts and utility poles. One proposed solution is to separate the transmitter and receiver devices and the base station controller from the antenna section in each radio base station, and to locate the transmitter and receiver devices and the base station controllers in a central base station, and the antenna sections in the radio base stations with the transmitter and receiver devices and the base station controllers being connected to the antenna sections through optical fibers (see, for example, OPTICAL FIBER TRANSMISSION 0F RF SIGNAL FOR MICROCELLULAR MOBILE COMMUNICATION SYSTEMS, SAT90-32, RCS90-12, by Shibutani et al., Radio Communication System Research Group, the Institute of Electronics, Information and Communication Engineers). According to the proposed system, since radio signals are transmitted through optical fibers from the central base station to the radio base stations, the radio base stations in each cell can be reduced in size and cost, making it possible to realize a microcellular mobile communication system based on microcells. However, a microcellular mobile communication system employing optical fiber feeders normally requires that the central base station and each of the radio base stations be connected to each other by optical fibers. Therefore, as the microcells decrease in size and increase in number, the number of optical fibers required becomes so enormous that sufficient optical fibers may not be available depending on the conditions in which optical fiber cables are actually installed.